Persistent phosphorescence was discovered in the 11th century in China and Japan and in the 16th century in Europe (Shionoya, S. (1998) in Phosphor Handbook, Shionoya, S; and Yen, W. M; (eds.), CRC Press, Inc., Boca Raton, New York, p. 3). The phenomenon involves two kinds of active centers, emitters and traps. Emitters are centers capable of emitting radiation after excitation of the center. Traps do not emit radiation, but store excitation energy and release it gradually to the emitter. Emitter centers can be created through the addition of activators, i.e., small amounts of intentionally added impurity atoms or ions, to the host matrix. Co-activators are additional, intentionally-added impurity ions which may affect (improve or modify) the emission of an activator. For example, a co-activator can be added to form trapping centers that can increase the persistence time of the phosphor.
Persistence of phosphorescence is measured herein as persistence time, which is the time, after discontinuing irradiation, that it takes for phosphorescence of a sample to decrease to the threshold of eye sensitivity. This threshold is the signal level of emission intensity that an unaided eye can perceive in the dark. Persistence times are assessed by following phosphorescence intensity as a function of time. Measurement comparisons of persistence times must be performed under identical conditions using the same detection systems.
Persistent phosphors are a luminescent material to show bright afterglow, in dark after charged by UV light. Various persistent phosphors in the visible have been developed. These include (a) sulfides, such as ZnS:Cu (green), ZnS:Cu, Co (green) and ZnS:Mn,Cu (green), CaS:Bi (460 nm, blue) and CaS:Eu,Tm (450 nm, red) developed before the sixties; (b) aluminates, such as CaAl2O4:Eu2+,Nd3+ (F. C. Palilla, A. K. Levine and M. R. Tomkus, Fluorescent Properties of alkaline earth aluminate of the type M2Al2O4 activated by divalent europium, J. Electrochem. Soc. 115 (6) (1968) 642-644), SrAl4O7:Eu2+,Pr3+/Dy3+ (W. M. Yen, W. Jia, L. Lu, H. Yuan, U.S. Pat. No. 6,117,362; 2000), SrAl2O4:Eu2+,Dy3+ (W. M. Yen, W. Jia, L. Lu and H. Yuan, U.S. Pat. No. 6,267,911 B1, 2001; H. Lange, U.S. Pat. No. 3,294,699, 12/1996, Murayama et al. U.S. Pat. No. 5,424,006; T. Matsuzawa, Y. Aoki, N. Takeuchi and Y. Murayama, A new long persistent phosphor with high brightness SrAl2O4:Eu2+,Dy3+, J. Electrochem. Soc. 143 (8) (1996) 2670-2673); those phosphors have emission in the violet (440 nm), green-blue (490 nm) and green (520 nm), respectively; and (c) silicates, such as Sr2MgSi2O7:Eu2+,Dy3+ with phosphorescence at 470 nm (Zhiguo Xiao and Zhiqiang Xiao, U.S. Pat. No. 6,093,346, 2000), SrMgSi2O6:Eu2+,Nd3+ (470 nm, D. Jia, W. Jia, Y. Jia, to be published in J. Appl. Phys.), and Ca3MgSi2O8:Eu2+,Dy3+ with afterglow band at 475 nm (Yuanhua Lin, Zhongtai Zhang, Zilong Tang, Xiaoxin Wang, Junying Zhang, Zhishan Zheng, J. Eur. Ceram. Soc. 21 (2001) 683). From the above list, it can be seen that all the emission of the persistent phosphors developed up to now are in the visible. The longest wavelength is in red near 650 nm (CaS:Eu2+,Tm3+). These visible persistent phosphors have been widely used for security signs, indicators of control panels, and so on.
In the recent technical development, persistent phosphors in infrared or near infrared have gained attention because of military and security applications. Unfortunately, up to now, no such persistent phosphors are available in market.
It has been reported that some lanthanum gallates showed strong emission in the infrared and the single crystals were used for tunable infrared lasers. For example, La3Ga5SiO14:Cr3+[A. A. Kaminskii, A. P. Shkadarevich, B. V. Mill, V. G. Koptev and A. A. Demidovich, Wide-band tunable stimulated emission from a La3Ga5SiO14:Cr crystal, Inorg. Mater. (USSR) 23 (1987) 618; S. T. Lai, B. H. T. Chai, M. Long and M. D. Shinn, Room temperature near-infrared tunable Cr:La3Ga5SiO14 laser, IEEE J. Quantum Electron. Qe-23 (1987) 24]; La3Ga5GeO14—Cr [A. A. Kaminskii, A. P. Shkadarevich, B. V. Mill, V. G. Koptev, A. V. Butashin, and A. A. Demidovich, Tunable stimulated emission of Cr3+ ions and generation frequency self-multiplication effect in acentric crystals of La-gallogermanate structure, Inorg. Mater. (USSR), 24 (1988) 579]; La3Ga5.5Nb0.5O14:Cr and La3Ga5.5Ta0.5O14:Cr [A. A. Kaminskii, A. P. Shkadarevich, B. V. Mill, V. G. Koptev, A. V. Butashin and A. A. Demidovich, Wide-band tunable stimulated emission of Cr3+ ions in the trigonal crystal La3Ga5.5Nb0.5O14, Inorg. Mater. (USSR) 23 (1987) 1700]. However, no afterglow has been reported in the luminescent crystals.
In addition, the persistent time of infrared phosphors have to be redefined. Traditionally, the persistent time of visible phosphors is estimated based on perceivable luminous level of the afterglow by human eyes. For infrared persistence phosphors, such definition is no longer valid. The persistent time then should be determined by sensitivity of the detection systems and UV pump sources.